Recent developments have occurred in the field of very small switches having moving liquid metal-to-metal contacts and that are operated by an electrical impulse. That is, they are actually small latching relays that individually are SPST or SPDT, but which can be combined to form other switching topologies, such as DPDT. (Henceforth we shall, as is becoming customary, refer to such a switch as a Liquid Metal Micro Switch, or LIMMS.) With reference to FIGS. 1-4, we shall briefly sketch the general idea behind one class of these devices. Having done that, we shall advance to the topic that is most of interest to us, which is an improved technique for forming the needed channels and cavities of such switches fabricated on a substrate.
Refer now to FIG. 1A, which is a top sectional view of certain elements to be arranged within a cover block 1 of suitable material, such as glass. The cover block 1 has within it a closed-ended channel 7 in which there are two small movable distended droplets (12, 13) of a conductive liquid metal, such as mercury. The channel 7 is relatively small, and appears to the droplets of mercury to be a capillary, so that surface tension plays a large part in determining the behavior of the mercury. One of the droplets is long, and shorts across two adjacent electrical contacts extending into the channel, while the other droplet is short, touching only one electrical contact. There are also two cavities 5 and 6, within which are respective heaters 3 and 4, each of which is surrounded by a respective captive atmosphere (10, 11) of a suitable gas, such as N2. Cavity 5 is coupled to the channel 7 by a small passage 8, opening into the channel 7 at a location about one third or one fourth the length of the channel from its end. A similar passage 9 likewise connects cavity 6 to the opposite end of the channel. The idea is that a temperature rise from one of the heaters causes the gas surrounding that heater to expand, which splits and moves a portion of the long mercury droplet, forcing the detached portion to join the short droplet. This forms a complementary physical configuration (or mirror image), with the large droplet now at the other end of the channel. This, in turn, toggles which two of the three electrical contacts are shorted together. After the change the heater is allowed to cool, but surface tension keeps the mercury droplets in their new places until the other heater heats up and drives a portion of the new long droplet back the other way. Since all this is quite small, it can all happen rather quickly; say, on the order of a millisecond, or less. The small size also lends itself for use amongst controlled impedance transmission line structures that are part of circuit assemblies that operate well into the microwave region.
To continue, then, refer now to FIG. 1B, which is a sectional side view of FIG. 1A, taken through the middle of the heaters 3 and 4. New elements in this view are the bottom substrate 2, which may be of a suitable ceramic material, such as that commonly used in the manufacturing of hybrid circuits having thin film, thick film or silicon die components. A layer 14 of sealing adhesive bonds the cover block 1 to the substrate 2, which also makes the cavities 5 and 6, passages 8 and 9, and the channel 7, each moderately gas tight (and also mercury proof, as well!). Layer 14 may be of a material called CYTOP (a registered trademark of Asahi Glass Co., and available from Bellex International Corp., of Wilmington, Del.). Also newly visible are vias 15-18 which, besides being gas tight, pass through the substrate 2 to afford electrical connections to the ends of the heaters 3 and 4. So, by applying a voltage between vias 15 and 16, heater 3 can be made to become very hot very quickly. That in turn, causes the region of gas 10 to expand through passage 8 and begin to force long mercury droplet 12 to separate, as is shown in FIG. 2. At this time, and also before heater 3 begins to heat, long mercury droplet 12 physically bridges and electrically connects contact vias 19 and 20, after the fashion shown in FIG. 1C. Contact via 21 is at this time in physical and electrical contact with the small mercury droplet 13, but because of the gap between droplets 12 and 13, is not electrically connected to via 20.
Refer now to FIG. 3A, and observe that the separation into two parts of what used to be long mercury droplet 12 has been accomplished by the heated gas 10, and that the right-hand portion (and major part of) the separated mercury has joined what used to be smaller droplet 13. Now droplet 13 is the larger droplet, and droplet 12 is the smaller. Referring to FIG. 3B, note that it is now contact vias 20 and 21 that are physically bridged by the mercury, and thus electrically connected to each other, while contact via 19 is now electrically isolated.
The LIMMS technique described above has a number of interesting characteristics, some of which we shall mention in passing. They make good latching relays, since surface tension holds the mercury droplets in place. They operate in all attitudes, and are reasonably resistant to shock. Their power consumption is modest, and they are small (less than a tenth of an inch on a side and perhaps only twenty or thirty thousandths of an inch high). They have decent isolation, are reasonably fast with minimal contact bounce. There are versions where a piezo-electrical element accomplishes the volume change, rather than a heated and expanding gas. There also exist certain refinements that are sometimes thought useful, such as bulges or constrictions in the channel or the passages. Those interested in such refinements are referred to the Patent literature, as there is ongoing work in those areas. See, for example, U.S. Pat. No. 6,323,447 B1.
To sum up our brief survey of the starting point in LIMMS technology that is presently of interest to us, refer now first to FIG. 4 and then to FIG. 5. In FIG. 4 there is shown an exploded view 32 of a slightly different arrangement of the parts, although the operation is just as described in connection with FIGS. 1-3. In particular, note that in this arrangement the heaters (3, 4) and their cavities (5, 6) are each on opposite sides of the channel 7. Another new element to note in FIG. 4 is the presence of contact electrodes 22, 23 and 24. These are (preferably thin film) depositions of metal that are electrically connected to the vias (19, 20 and 21, respectively). They not only serve to ensure good ohmic contact with the droplets of liquid metal, but they are also regions for the liquid metal to wet against, which provides some hysteresis in the pressures required to move the droplets. This helps ensure that the contraction caused by the cooling (and contraction) of the heated (and expanded) operating medium does not suck the droplet back toward where it just came from. The droplets of liquid metal are not shown in the figure.
If contact electrodes 22-24 are to be produced by a thin film process, then they will most likely need to be fabricated after any thick film layers of dielectric material are deposited on the substrate (as will occur in connection with many of the remaining figures). This order of operations is necessitated if the thick film materials to be deposited need high firing temperatures to become cured; those temperatures can easily be higher than what can be withstood by a layer of thin film metal. Also, if the layer of thin film metal is to depart from the surface of the substrate and climb the sides of a channel, then it might be helpful if the transition were not too abrupt.
FIG. 5 is a simplified exploded view 25 of a LIMMS device whose heater cavities, liquid metal channel and their interconnecting passages are formed in a layer of dielectric material between two substrates, instead of being recesses in a cover block. The figure shows a portion of a substrate 26, which may be of ceramic or glass, and which serves as a base upon which to fabricate the LIMMS device. Various metal conductors 27-31, which may be of gold, are deposited on the top surface of the substrate 26, or they may be what remains from a patterned removal of an entire metal sheet originally present on the surface of the substrate. The latter case cooperates nicely in instances where some of the conductors are to be co-planar transmission lines formed with the presence of a ground plane. Mercury amalgamates with gold, however, and if enough mercury is present, will dissolve it. It is therefore desirable to protect the gold with a covering of another metal, such as chromium or molybdenum. (Owing to the possibility of mercury smears during assembly, a complete over-covering of all the gold is more desirable than simply covering the exposed pads where the droplet or slug of mercury might be expected to touch the gold during normal operation.) In the figure, conductors 27 and 28 are drive lines for heater resistors 34 and 35, respectively. Conductors 29, 30 and 31 are switched signal lines that might also be parts of a controlled impedance transmission line structure.
Now note patterned layer 36. It is applied over the various conductors 27-31, and may be of KQ 150 or KQ 115 thick film dielectric material from Heraeus, or the 4141A/D thick film compositions from DuPont. These are materials that are applied as pastes and then cured under heat at prescribed temperatures for prescribed lengths of time. Depending upon the particular material, they may be applied as an undifferentiated sheet, cured and then patterned (say, by laser or chemical etching) or they may be patterned upon their initial application (via a screening process). In any event, the patterning produces the heater cavities 44 and 45, the liquid metal channel 46 and their interconnecting passages.
The conventional thick film processes used to print patterned layers of the dielectric material allows considerable control over the finished thickness of a cured layer of dielectric material (say, in the range of five to ten thousandths of an inch), and achieving sufficient uniformity of thickness is not a major difficulty. However, there are limits to how thin and how thick an uncured printed layer can be, and it may be necessary to apply (print) multiple layers to achieve a particular overall depth for layer 36. For the KQ material that is to be printed on using a fine mesh (screen) of stainless steel, a printed uncured layer is on the order of one to two thousandths of an inch in thickness. The KQ material shrinks in thickness by an amount of about thirty percent during the curing process. It is possible to print several uncured layers, one on top of the other, and then fire the whole works, or, the application sequence could be printxe2x80x94firexe2x80x94printxe2x80x94fire . . . , or even printxe2x80x94print . . . printxe2x80x94firexe2x80x94printxe2x80x94print . . . During the firing for curing the steep side walls and relatively sharp edges possible for the uncured printed layers become sloped and rounded, respectively. The resulting trapezoidal cross-sectional shape of the liquid metal channel 46 may be a significant influence in determining a desired thickness for layer 36. In this connection, the view shown in FIG. 5 is a considerable simplification, in that, for simplicity of the drawing, the heater cavities 44 and 45, liquid metal channel 46, and their interconnecting passages (not numbered in FIG. 5, but are shown as 8 and 9 in FIGS. 1 and 2) are all depicted as having steep side walls and sharp edges. It makes the basic subject matter of the drawing much easier to appreciate. When using printed KQ, however, the actual situation is much close to what is shown in FIGS. 6-9. Note the sloping side walls of the various patterned layers of dielectric material. Steep sidewalls and sharp edges are not necessarily bad, and can be obtained with other fabrication techniques, although that may also have an effect on the method used to create metalized regions, such as 41-43 that are to ascend such steep side walls.
Once layer 36 has been formed and patterned, metallic regions 41-43 are deposited. These correspond to metallic contacts 22-24 of FIG. 4, and serve to improve electrical contact with the liquid metal and to provide a surface that can be wetted by the liquid metal (for latching). Regions 41-43 may be deposited by thin film techniques, in which case it may be important that any high temperature firings needed to cure the dielectric layer 36 have already been performed.
If desired, a strip of metal 37 may be applied around the perimeter of the LIMMS device. Such a strip 37 is part of an hermetic seal with a cover plate 38 and is formed of solder or glass frit. The hermetic seal may also involve there being a beveled edge 39 along the perimeter of the cover plate 38. Cover plate 38 is preferably of ceramic, although one could use glass, as well. On the underside of the cover plate is applied a patterned layer 40 of adhesive, such as CYTOP. The patterning of the adhesive layer 40 matches that of the dielectric layer 36 that it is to mate against, and is shown by the dotted lines. Also shown as dotted lines are metalized regions 47, 48 and 49 that correspond to the regions 41-43 formed in the channel 46. Metalized regions 47-49 offer additional surface for wetting at the various locations of the liquid metal, and may also be deposited by thin film techniques.
To assemble the LIMMS shown in view 25 of FIG. 5, the channel 46 would receive its droplets of liquid metal (not shown) and, while in an atmosphere of a suitable gas, such as N2, the cover plate 38 would be affixed against the substrate 26 bearing the patterned layer 36 of dielectric material. Then the hermetic seal would be formed.
We are always interested in techniques that improve device capability, reduce device fabrication cost, reduce the costs associated with connecting the device to a surrounding circuit, reduce device power consumption or increase the reliability of the device and its various interconnections with other circuitry. The speed of operation and power consumption of a LIMMS device would be favorably affected if more of the operating gas were in contact with the heater resistor and if less of the heater resistor""s heat were captured by the substrate. Heater resistors that are affixed directly to traces or pads formed on the substrate are very close to the substrate, reducing the resistor area available to heat the gas and waste power by heating the substrate. Moreover, forming recesses in sheet of ceramic or glass is an onerous task, and one that may involve nasty chemicals that are not easily handled. Also, it may be that such forming requires process capabilities that not otherwise needed, so that if another existing process could be used instead, a certain simplification in manufacturing logistics is obtained. And there is the promise that if an existing process is re-used, the resulting structure will have thermal expansion characteristics that are quite compatible with the other structures. For these reasons, we should like to re-visit how the heaters are mounted, and perhaps get rid of the recesses dug into the cover block. The use on the bottom substrate of a patterned layer of dielectric forming cavities, channels and interconnecting passages is an attractive starting point. But then what?
An attractive solution to the problem of efficient fabrication of the channels and cavities in a LIMMS device is to form them as matching upper and lower portions each created as a patterned layer of thick film dielectric material deposited on a respective upper or lower substrate. The two portions are adhered together by a patterned layer of adhesive, and hermetically sealed around an outer perimeter. The heater resistors are mounted atop the lower layer, thus suspending them away from that substrate and exposing more of their surface area. Vias can be used to route the conductors for the heaters and the switched signal contacts through the lower substrate to cooperate with surface mount techniques using solder balls on an array of contact pads. Such vias are not normally hermetic, but can be made so by their placement within the patterned layers of dielectric material. If desired, the upper substrate and its patterned dielectric layer could be replaced by a conventional flat substrate that has had recesses formed therein.
This plan depends upon the use of a suitable dielectric material, which must be strong, adheres well to the substrate, is impervious to contaminants, is capable of being patterned, and if also desired, which can be metalized for soldering. It should also have well controlled and suitable properties as a dielectric. Given a choice, a lower dielectric constant (K) is preferable over a higher one. Suitable thick film dielectric materials that may be deposited as a paste and subsequently cured include the KQ 150 and KQ 115 thick film dielectrics from Heraeus and the 4141A/D thick film compositions from DuPont.